1. Field of the Invention
This invention relates to fuel ignition systems of the pilot ignition type and more particularly, to a control circuit for use in such systems for providing an interlock on start-up under certain failure conditions.
2. Description of the Prior Art
In known fuel ignition systems of the pilot ignition type, a pilot valve is operated in response to the closure of thermostatically controlled contacts to supply fuel to a pilot outlet for ignition by a suitable igniter to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve which supplies fuel to a main burner apparatus for ignition by the pilot flame.
Typically, the operation of the main valve is controlled by a relay of the flame sensing circuit which has normally open contacts connected in the energizing path for the main valve to maintain the main valve deenergized until a pilot flame is established. When a pilot flame is established, the flame sensing circuit energizes the relay which closes its contacts to connect the main valve to an energizing circuit to permit the main valve to operate.
After the heating demand has been met, the thermostatically controlled contacts open to effect deenergization of the fuel valves and cause the flame to be extinguished. The flame sensing circuit responsively causes the relay to be deenergized, opening its contacts to disconnect the main valve from the energizing circuit in preparation for the next heating cycle. However, should the relay contacts which control the energization of the main valve become welded together following a successful ignition cycle, then, when the relay is deenergized, the main valve remains connected to the energizing circuit and will be energized when the thermostatically controlled contacts close in response to the call for heat, even though a pilot flame is not established. Similarly, the main valve will also be connected to the energizing circuit for a circuit failure which permits the relay of the flame sensing circuit to be energized in the absence of a pilot flame. For such failure conditions, both the pilot valve and the main valve will be energized when the thermostatically controlled contacts close, permitting fuel to emanate from the pilot outlet and the main burner, an undesireable condition.
Various interlock arrangements have been proposed in the prior art as exemplified by the U.S. Pat. Nos. 3,449,055 to L.C. Blackett, 3,644,074 to P.J. Cade and 3,709,783 to J.S. Warren, in which the energization of the fuel valves of the system is dependent upon the sequential operation of relays. In the systems disclosed in the patents referenced above, the energization of the pilot valve is effected in response to the operation of a control relay which can be energized only if the flame relay is deenergized. Thereafter, the energization of the main valve is effected in response to the operation of a flame relay when a pilot flame is established, but only if the control relay is energized.
While such interlock circuits guard against the welded contact failure referred to above, it appears that the control (or flame) relay may be energized inadvertantly following a failure of a solid state control device of the electronic circuits, allowing the main valve to operate in the absence of a pilot flame. Also, in the patented systems, the flame sensing circuit is energized in response to the operation of a control device, such as a thermostat. Thus, in the event of a leak condition for a fuel valve of the system, which permits a flame to remain establish after the system is deactivated, it would appear that under certain failure conditions, the flame sensing circuit would be ineffective to lockout the system before the control relay operated.
In the U.S. Pat. No. 3,840,322 to Philip J. Cade, there is disclosed an automatic fuel ignition control system which effects lock out of the system whenever a flame is provided at a burner apparatus before the end of an ignition timing interval. However, the system will also be locked out following a loss of flame during a heating cycle or in the event of a line voltage interruption of a very short duration, wherein the pilot frame is not extinguished before power is restored.
In my U.S. Pat. Application Ser. No. 621,670, filed Oct. 14, 1975 and now U.S. Pat. No. 4,035,134, there is disclosed a proven pilot fuel ignition system including a control arrangement which provides an interlock on start-up to prevent the energization of fuel valves of the system under certain failure conditions. The control arrangement also detects a leak condition for pilot and main valves of the system, effecting shut down of the system for a leak condition for either valve, but permitting recycling of the system following a momentary power loss or a flame out condition.
The control arrangement includes a delay circuit which effects operation of a control relay to allow energization of the pilot valve and a flame sensing circuit ater a first delay following the activation of the system. If a flame is established at the time the flame sensing circuit is energized, the flame sensing circuit causes the system to be locked out. Such timing control is afforded by a timing circuit of the flame sensing circuit which requires absence of a flame for a time after the flame sensing circuit is energized before a flame relay of the flame sensing circuit is allowed to operate. Under normal conditions, the delay circuit is disabled under the control of the flame relay and effects energization of the main valve after a second delay interval after the pilot flame is established. The flame sensing circuit prevents operation of the main valve if a main burner flame is sensed during the second delay interval. An interlock circuit prevents enabling of the delay circuit and the control relay at start up for certain failure conditions, including welded contacts of the flame relay.